Dirt accumulates inside the cork barrel.
Clean out this dirt with a small brush, feather, or pipe cleaner. Use some trombone oil to loosen. If you don't keep this dirt out of
the cork barrel, the outside slide will gradually draw it out and it will
foul the action. Often this is the cause of a certain mysterious slide
trouble. After the trombone is thoroughly cleaned and oiled, the slide
action is fine for awhile. Then it begins to drag and bind. The reason is
this: the dirt is being loosened and dragged out of the cork barrel. If your
trombone has a spring stop instead of the cork stop, the same precautions
apply. See Fig. 20 and Fig. 21.